The present invention relates to a high-pressure sodium lamp.
Incandescent lamps emit light of warm colors, have excellent color rendition and are low in cost so that they find a wide application in indoor lighting. However, with the increasingly serious energy problem, the low efficiency of incandescent lamps has become a problem. There is a strong demand for the development of discharge lamps which are compact in size yet capable of generating lumens equivalent to those of incandescent lamps of the ratings from 60 to 200 watts. To this end, there have been devised and demonstrated various types of high-pressure sodium lamps with high color rendition at the ratings of 150 to 400 watts. However, when such lamps are fabricated based upon the ordinary design criteria, there arise the problems that the lamp efficiency is extremely low and fabrication costs are relatively high.
In order to improve the luminous efficacy, single-crystalline alumina which has a higher degree of transmittance than conventional alumina ceramic is used in the fabrication of arc tubes. In this case, end caps made of alumina are fitted into the ends of an arc tube and metal tubes, each having an electrode, are mounted in the end caps so as to gas-tightly seal the ends of the arc tube. As a result, of the construction cracks are propagated at the sealed ends of the arc tube made of single-crystalline alumina and in the end caps.